The present invention generally relates to expandable intraluminal vascular grafts, most often referred to as stents, and more particularly pertains to biodegradable stents which completely or partially degrade or are bioabsorbed over a period of time after deployment.
Stents are used to maintain the patency of vessels in the body. They are typically advanced through the vasculature to the deployment site while in a contracted state where they are then expanded to engage the vessel walls and thereby establish a flowpath therethrough. A stent can be moved along a guide wire previously positioned in the vessel and then expanded by the inflation of a balloon about which such stent is disposed. Subsequent deflation of the balloon and removal of it along with the guidewire leaves the stent in place and locked in its expanded state. It has been found that the continued exposure of a stent to blood can lead to undesirable thrombus formation, and the presence of a stent in a blood vessel can over time cause the blood vessel wall to weaken, which creates the potential for an arterial rupture or the formation of aneurisms. A stent can also become so overgrown by tissue after its implantation that its usefulness may be substantially diminished while its continued presence may cause a variety of problems or complications.
In certain situations it is therefore desirable for the stent to be biodegradable or bioabsorbable so as to curtail the adverse risks that would otherwise be associated with the stent""s continued presence once its usefulness at the treatment site has ceased or has at least become substantially diminished. To such end, some stents have heretofore been wholly constructed of materials that are biodegradable or bioabsorbable. It is of course necessary to select a material that while biodegradable is nonetheless biocompatible and additionally, has the physical properties necessary to properly serve its function as a stent. Such physical properties include, among others, sufficient strength to support the loads a particular stent is to be subjected to in its function as a splint, the radial flexibility necessary for it to undergo expansion, longitudinal flexibility to allow it to be advanced through a contorted vasculature and conceivably to adapt to a non-linear deployment site.
Such characteristics have heretofore been achieved with the use of certain polymer materials such as polylactic acid, polylactic acid-glycolic acid copolymer, and polycaprolactone. However, all such previously known biodegradable/bioabsorbable stents exhibit bulk erosion and are as a consequence prone to break up into large particles as the matrix breaks down. Additionally, such materials have also been used as stent coatings to gradually release pharmacological agents that are infused throughout the coating. However, the bulk erosion to which such materials are inherently prone to can cause the coating to flake off or otherwise become detached. Should such large particles actually become dislodged before becoming completely degraded, they could be washed downstream and cause emboli.
A biodegradable stent is therefore needed that is initially capable of providing the necessary structural support to a body lumen and then gradually and completely degrades or is absorbed in a manner that precludes a break-up into large particles. Similarly, a biodegradable coating is needed that is not prone to flaking or breaking up into large particles. By preventing the break-up of the stent or of the stent coating into large particles that may subsequently be swept downstream, the potential for embolic complications is thereby avoided.
The present invention provides a stent or optionally, a stent coating which degrades in a very controlled and uniform manner so as to substantially preclude the possibility of sizeable particles becoming detached and possibly causing embolic problems downstream. This is achieved by employing a material in the construction of the entire stent or in the coating of the stent that erodes in a very controlled manner. Such material is selected for its strength characteristics as well as its tendency to erode from the surface inwardly rather than being subject to bulk erosion. By incorporating pharmacological agents within the material, the stent or stent coating not only eventually vanishes from within the body lumen in which it was implanted but additionally dispenses the incorporated drug in a gradual manner.
Materials that exhibit the desired surface eroding characteristics without being subject to bulk erosion include polymers wherein the degradation rate of the matrix is faster than the rate of water penetration into the interior of the polymeric mass. Such polymers are hydrophobic but have water-labile linkages interconnecting the monomers. The hydrophobic property precludes water from penetrating into the interior of the polymer while water labile linkages nonetheless subject the surface to gradual erosion. As a result, the stent gradually degrades from the surface inwardly, substantially without the risk of large particles becoming dislodged.
While hydrophobic polymers with water-labile linkages are known, their limited strength and processing capabilities have restricted their usage to passive devices that neither perform a structural function nor are subject to stress or distortion. Drugs infused throughout such material implanted in the body in the form of a tablet or other shape are gradually released as the polymer degrades. As such, these surface degrading polymers have functioned as an effective drug delivery vehicle. The use of such polymers in stent applications has however been precluded as they are unable to support a lumen wall or remain attached to a stent as it undergoes deformation during its expansion.
The materials employed in either wholly forming a stent or in coating a stent in accordance with the present invention include hydrophobic polymers having water-liable linkages connecting the monomers that are fortified with the incorporation of ester or imide bonds. Examples of such polymers include polyanhydrides and polyorthoesters. Additionally, by employing such polymers in stent applications, a single device can be called upon to provide the necessary support to a body lumen and simultaneously dispense a pharmacological agent in a controlled manner.
These and other features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiments which illustrate by way of example the principles of the invention.